1. Field of the Invention
The present invention relates to a tyre for motor vehicles.
In particular, the present invention relates to high performance tyres such as, for example, tyres designed for high-powered cars or, more generally, tyres intended for applications involving high operating speeds.
In more details, the present invention relates to “HP” (High Performance) or “UHP” (Ultra High Performance) tyres.
2. Description of the Related Art
High speed running, e g. higher than 200 km/h, generates remarkable centrifugal forces at the tread of the tyre due to the rotation thereof.
Said centrifugal forces cause the tyre tread to swell outwardly resulting in the lifting of the tyre tread in the radial direction. This is known as “lifting phenomenon”.
This phenomenon should be suitably controlled and limited as much as possible since it negatively affects the tyre behaviour.
For instance, since vehicle electronic systems (e.g. Anti-lock Braking System (ABS), Electronic Stability Program (ESP), traction distribution on the four driving wheels) are traditionally correlated to the variation of the wheel rolling height and are set to a predetermined range thereof, in the case the lifting phenomenon gives rise to an important swelling of the tyre—so that the wheel rolling height falls away from said range—a correct functioning of the abovementioned vehicle electronic systems is no more guaranteed.
Furthermore, in the case the lifting phenomenon is not suitably controlled and limited, an important and different variation of the wheel rolling height can occur in the tyres of the same vehicle so that the latter is equipped with tyres having even very different behaviours from each other.
Moreover, in the case the tyre deformation due to the lifting phenomenon is considerably high, a plurality of further drawbacks can occur.
For instance, the lifting in the radial direction of the crossed belt plies, especially in correspondence of the axial edges thereof, can arise thereby causing the detachment of the belt plies from the carcass; an uneven weariness of the tread band and thus a remarkable decrease of the durability thereof at high speeds can occur; undesired vibrations of the tyre resulting in negatively affecting the ride comfort and remarkably increasing the noisiness of the tyre at high speeds can be promoted.
In order to at least partially solve the abovementioned problems caused by the lifting phenomenon, a belt layer is generally positioned radially external to the crossed belt plies in order to constrain the latter so as to limit the lifting thereof. Generally, said belt layer is provided with low elastic modulus organic fibre cords, e.g. nylon cords, or high elastic modulus organic fibre cords, e.g. aromatic polyamide cords, which are disposed in a substantially circumferential direction with respect to the equatorial plane of the tyre.
Document EP-335,588 discloses a tyre, particularly suitable for high speed passenger cars, comprising a band disposed radially outside the tyre belt, said band comprising a ply composed of at least one cord wound spirally and continuously in the circumferential direction of the tyre at 0 to 3 degrees to the equator of the tyre. The cord of said ply is a hybrid cord comprising a high elastic modulus filament and a low elastic modulus filament twisted together, the hybrid cord having a low elastic modulus in a low elastic modulus zone between zero elongation and a predetermined specific elongation in the range of 2-7% and a high elastic modulus in a high elastic modulus zone above said predetermined specific elongation of the cord. The low and the high elastic moduli change at a transitional point derived from the load elongation curve of the hybrid cord, being the intersecting point of a line orthogonal to the elongation axis passing through the intersection of the tangent to the elongation curve at zero elongation and the tangent to the elongation curve at the break point.
EP-790,143 discloses an improved tyre belt structure capable of improving high speed durability and cornering performance, said belt structure comprising at least two crossed plies of high elastic modulus and a band disposed radially outside said crossed plies, said band comprising a full-width band, which is made of low tensile elastic modulus organic fibre cords arranged substantially parallel to the tyre equator, and a pair of axially spaced edge bands, which are made of high modulus organic fibre cords arranged substantially parallel to the tyre equator. The width of each of the edge bands is between 13% and 26% of the width of the ground contacting area of the tread portion. According to said document the edge bands are provided in order to prevent the lifting of the belt structure and the occurring of the belt edge separation failure when the tyre is subjected to a remarkable centrifugal force during high speed running thereof.
EP-571,204 discloses a pneumatic vehicle tyre comprising a breaker and a bandage of reinforcement plies which extend substantially in the circumferential direction of the tyre. The bandage reinforcement plies consist of overlapping strips in the tyre shoulder regions and are made of only a hybrid material consisting of polyaramid and polyamide. A further middle bandage reinforcement ply of conventional type is provided between two bandage reinforcement plies and is made of polyamide. According to said document the belt structure disclosed therein improves the pneumatic tyre with respect of its high speed running performance, its flat-spot behaviour and its wear behaviour.